A. Field of the Invention
This invention relates generally to the electronic signalling art and, in particular, to an improved encoder/decoder system suitable for tone coded squelch systems.
B. Description of the Prior Art
As a result of the extensive use of radio communications channels presently available, it has become common practice especially in heavily populated metropolitan areas to have several communications systems operating on a common radio channel. Operators whose radios are not equipped with some form of tone squelch must listen to all conversation on the channel from transmitters that are within range. This is both annoying and fatiguing for the operators. The use of tone squelch to alleviate the annoyance is a common solution to the problem. A standard for sub-audible continuous tone control squelch systems (CTCSS) is covered in Electronic Industries Association (EIA) Standard RS 220-A, published in March, 1979.
Basically, such systems operate by equipping each radio transmitter in the particular radio system with a tone encoder operating on one of the 33 standard EIA channels between 67.0 and 250.3 Hz. When the transmitter is keyed, the operator's voice modulates the transmitter in the 300-3000 Hz range and simultaneously with the voice, the sub-audible tone modulates the transmitter as well.
Receivers within the radio system are equipped with a frequency selective device (tone decoder) tuned to the same tone frequency as the tone encoder within the transmitters of the system. This tone decoder normally holds the receiver in the squelched mode. If the received signal is not modulated with a tone or is modulated with a tone that is not on the decoder frequency, then the receiver will remain squelched and the operator will not hear the interference. If a transmission is received having a tone corresponding to the frequency of the tone decoder, the receiver will be unsquelched and the operator will receive the message.
Many of the tone squelch systems now being manufactured employ vibrating mechanical resonant reeds. Resonant reed systems suffer from a number of problems. In order to reprogram such a system, the resonant reed must be replaced by a reed of another frequency. Long lead times are involved as service organizations rarely stock the 33 types of encoding and decoding reeds required for this type of change. In addition, vibrating mechanical reeds suffer from short life and false operation when subject to vibration encountered in vehicular two-way radio service.
Solutions for these and other problems are provided by the instant invention which is an integratable, solid state, programmable encoder/decoder tone coded squelch system. The frequency of the tone to be detected and the tone encoder is programmable and can be specified with sufficient accuracy to meet EIA standards for sub-audible continuous tone controlled squelch systems.